|Hmmm, that would be highly unusual to be getting leaks from using fabric softener on fitted diapers just once. The fleece in pocket diapers would make them more vulnerable but even then I'd expect it would take more than just one wash. ME even says that those who really love their fabric softener can use it but just to do so sparingly.
Is someone helping you with the changes and maybe not changing as often? Or maybe if you're visiting you're busier than usual and haven't been changing as often?
In any case it's not a disaster, normal washing should remove the fabric softener pretty easily, especially if it was just the once. I wouldn't think you'd need to do anything special but if you'd like you could run them through an additional hot wash just to be safe...
Vinegar is a natural fabric softener, probably not to the extent that commercial fabric softeners are but it works for many. Unfortunetly I don't know of any diaper friendly fabric softener. Fabric softeners work by coating fabric with a waxy substance. It has to stay in fabric in order to work so it's only natural that it would build-up over time.